Description of the Prior Art
It is known that the frequency of an oscillator having a field effect transistor, FET, as its active element can be locked to the frequency of variations in the intensity of light in an optical signal injected into the active region of the FET that lies between its source and drain electrodes. For any given amount of modulated light intensity, there is a given bandwidth of frequencies, .DELTA.f, called the locking bandwidth, to which the oscillator can be locked. Whereas the locking bandwidth can be increased by increasing the peak modulated light intensity, a limit is reached when the active region of the FET becomes saturated. At the present time, the highest ratio of the locking bandwidth, .DELTA.f, to the free running frequency of an oscillator being locked is about 0.2%. Thus, in a situation where a large number of oscillators are to be locked to the same optical signal, it is essential that their free running frequencies be within .DELTA.f of the frequency of the optical signal for locking to occur. Unfortunately, however, this can be very expensive to obtain.
In an article entitled "Optical Injection Locking of FET Oscillators Using Fiber Optics", by D. C. Buck and M. A. Cross, that was published in a "Digest of Papers" for the IEEE MTT Symposium of 1986, a locking bandwidth of 5 MHz for an oscillator in the S band was said to have been achieved by injecting the optical signal into a FET via an optical fiber. This was achieved by using a lens to transform the cylindrical beam emerging from the fiber into an elliptical beam having its major axis aligned with length of the long narrow space between the source and drain electrodes of the FET.
In an article entitled "Optical Phase Control of an Optically Injection-Locked FET Microwave Oscillator", by Ronald D. Esman, Lew Goldberg and J. F. Weller, that was published in Vol. 37, No. 10 of the IEEE Transactions On Microwave Theory and Techniques of October, 1989, lock-in ranges that are a small fraction of 1% are reported. Also described are two ways of improving the coupling of optical energy to a FET. In one, the passivation layer is made to be antireflective, but the optical signal is shown as passing through space. In the other, an optical fiber is inserted through the substrate into contact with the active region of the FET, but this requires special construction not found in FET's available on the market.